abyssal plain

noun1

1. •
   A large expanse of very flat and smooth ocean floor, usually found at depths of 4,600 to 5,500 meters (15,000 to 18,000 feet).

   • Innocuous as it sounds, this note was a starting gun for a resource race on the planet’s last vast frontier: the abyssal plains that stretch between continental shelves deep below the oceans. — 2021